I guess if you looked at previous 'results' and picked the time of a finisher exactly half way down, or halfway up the list (depending on whether you are of the 'half full' or 'half empty' mindset), you should have an average time.
Or then again ..... you could add all the times together for say 10 years, divide by the number of finishers ...........etc. etc.
Sorry Corniceman, I'm feeling a bit pedantic this evening